Genetic differentiation in spite of high gene flow in the dominant rainforest tree of southeastern Australia, Nothofagus cunninghamii

Abstract

Nothofagus cunninghamii is a long-lived, wind-pollinated tree species that dominates the cool temperate rainforests of southeastern Australia. The species' distribution is more or less continuous in western Tasmania but is fragmented elsewhere. However, it is unknown whether this fragmentation has affected the species' genetic architecture. Thus, we examined N. cunninghamii using 12 nuclear microsatellites and 633 individuals from 18 populations spanning the species' natural range. Typical of wind-pollinated trees, there was low range-wide genetic structure (F ST =0.04) consistent with significant gene flow across most of the species' range. However, gene flow was not high enough to overcome the effects of drift across some disjunctions. Victorian populations (separated from Tasmania by the 240 km wide Bass Strait) formed a genetic group distinct from Tasmanian populations, had lower diversity (mean allelic richness (Ar)=5.4 in Victoria versus 6.9 in Tasmania) and were significantly more differentiated from one another than those in Tasmania (F ST =0.045 in Victoria versus 0.012 in Tasmania). Evidence for bottlenecking was found in small populations that were at least 20 km from other populations. Interestingly, we found little divergence in microsatellite markers between the extremes of genetically based morphological and physiological altitudinal clines suggesting adaptive differentiation is strongly driven by selection because it is likely to be occurring in the presence of gene flow. Even though the cool temperate rainforests of Australia are highly relictual, the species is relatively robust to population fragmentation due to high levels of genetic diversity and gene flow, especially in Tasmania.